Field of the Invention
The present invention relates to an optical box of an optical scanning apparatus mounted on an image forming apparatus using an electrophotographic process and an optical scanning apparatus including the optical box.
Description of the Related Art
There are image forming apparatuses that form images using an electrophotographic process and are provided with an optical scanning apparatus. FIG. 9 is a cross-sectional view illustrating a configuration of an image forming apparatus 50 that includes a plurality of image forming portions and prints a color image on a sheet (also referred to as a “recording material”) using a known electrophotographic process. The image forming apparatus 50 shown in FIG. 9 includes four image forming portions containing yellow (Y), magenta (M), cyan (C) and black (K) toners respectively. An optical scanning apparatus 20 irradiates photosensitive drums 21Y, 21M, 21C and 21K with a laser beam based on image information transmitted from an image reading apparatus (not shown) or a personal computer (not shown). FIG. 10 is a perspective view illustrating a configuration of the optical scanning apparatus 20 mounted on the image forming apparatus 50. The optical scanning apparatus 20 includes one rotary polygon mirror 31 provided at a center of a casing 35. To achieve a size reduction of the image forming apparatus 50, a scheme is used in which the photosensitive drums 21 of the plurality of image forming portions are exposed using one rotary polygon mirror 31. Laser beams emitted from light source units 31a and 31b are deflected by the rotary polygon mirror 31. After that, the corresponding photosensitive drum 21 is exposed by each laser beam via a scanning optical system and a reflection mirror provided for each light source.
A rotation speed of the rotary polygon mirror 31 is set based on resolution, sheet conveying speed, rotation speed of the photosensitive member and the number of light emitting points from which light beams for exposing the photosensitive member is emitted. That is, the rotation speed of the rotary polygon mirror 31 differs depending on each product specification. When an optical scanning apparatus having the same structure is mounted on a plurality of image forming apparatuses with different levels of productivity, the rotation speed of the rotary polygon mirror needs to be set appropriately in accordance with the specification of each image forming apparatus. For example, for an image forming apparatus A having an output sheet count per minute (productivity) of 70 and an image forming apparatus B having an output sheet count per minute of 50, it is assumed that sheets are conveyed at the same sheet interval. In this case, the image forming apparatus A needs to be set to a greater sheet conveying speed and a greater rotation speed of the photosensitive member than those of the image forming apparatus B. At this time, if an optical scanning apparatus of the same structure is mounted on the image forming apparatus A and the image forming apparatus B, the rotation speed of the rotary polygon mirror needs to be set as follows so that the interval of scan lines formed in respective scanning cycles of respective light beams is set to be equivalent to the resolution. That is, the rotation speed of the rotary polygon mirror of the image forming apparatus A needs to be set to be greater than the rotation speed of the rotary polygon mirror of the image forming apparatus B.
In general, as the rotation speed of the rotary polygon mirror increases, a noise level (wind noise) caused by rotation of the rotary polygon mirror also increases. Examples of a technique for reducing the noise level include a method of providing a sound insulation wall inside a casing (optical box) of the optical scanning apparatus. For example, as shown in FIGS. 11A and 11B, the periphery of the rotary polygon mirror 31 of the image forming apparatus A is covered with sound insulation members 40 and 41. The technique thereby prevents noise from leaking to outside the casing of the optical scanning apparatus. The sound insulation members 40 and 41 are provided with a transparent window 43a and a transparent window 43b (transparent window 43b is positioned at the opposite side of transparent window 43a as similarly shown in the depiction of an embodiment at FIG. 1) to allow laser beams deflected by the rotary polygon mirror 31 to transmit therethrough. The sound insulation members 40 and 41 may also be provided with transparent windows 42a and 42b to allow laser beams emitted from the light source units 31a and 31b and directed toward the rotary polygon mirror 31 to transmit therethrough. No sound insulation member 40 is set up in the image forming apparatus B. Therefore, neither transparent window 43a nor 43b exists in the image forming apparatus B.
Adopting an optical scanning apparatus having the same structure other than the presence or absence of the sound insulation members 40 and 41 for the image forming apparatuses A and B results in the following problem. That is, under an influence of the transparent window 43 of the sound insulation member 40, optical misalignment (defocusing and conjugate point misalignment) as shown in FIGS. 12A and 12B occurs. Details of FIG. 9 to FIGS. 12A and 12B will be described later. To cope with such defocusing and conjugate point misalignment, for example, Japanese Patent Application Laid-Open No. 2001-249295 proposes a casing of an optical scanning apparatus capable of adjusting a mounting position of a rotary polygon mirror to enable adjustment of a light path length.
According to this prior art, since the position of the rotary polygon mirror differs depending on the presence or absence of the sound insulation member, a configuration is proposed in which a positioning hole for mounting the rotary polygon mirror in the casing of the optical scanning apparatus is replaceable. In this configuration, molding is performed by replacing pieces at positions for positioning the rotary polygon mirror depending on the presence or absence of the sound insulation member. This changes a relative positional relationship between the rotary polygon mirror and respective optical parts of a scanning optical system that guides the light beam deflected by the rotary polygon mirror to a photosensitive member. As a result, optical performance of the parts attached to the image forming apparatus of at least one specification deteriorates. Mounting an electrical correction processing unit on the image forming apparatus to compensate for deterioration of the optical performance may reduce the cost merit that the optical scanning apparatus is shared among image forming apparatuses of different specifications.
The present invention has been implemented in such circumstances, and it is an object of the present invention to secure stable optical performance irrespective of the presence or absence of a sound insulation member that covers the rotary polygon mirror.